Polyamide fibers have been mass-produced for clothing, industrial use and interior use because of their excellent fiber properties. However, polyamide fibers, particularly fibers obtained from nylon 6, 66, etc. are poor in dry touch as presented by polyesters used as general purpose fibers, and have a greasy touch. For reducing the greasy touch, the use of thick and thin yarns has been attempted.
For production of thick and thin polyamide fibers, as techniques for causing melt fractures by use of abnormal flow at the spinneret, those disclosed in Japanese Patent Publication (Kokoku) Nos. 42-22576 and 44-7744 are publicly known. Furthermore, Japanese Patent Publication (Kokoku) No. 44-15573 discloses a method of spinning in a stress range to cause melt fractures by mixing a polyamide based polymer with a material having poor compatibility with the polymer. Moreover, Japanese Patent Laid-Open (Kokai) No. 55-122017 discloses a thick and thin yarn made of a composition obtained by mixing a polyester and a polyamide. Still furthermore, Japanese Patent Laid-Open (Kokai) No. 58-36210 discloses a method comprising the steps of blend-spinning a polyamide and a thermoplastic polymer 80.degree. C. or higher in glass transition temperature, and drawing at a low ratio. However, all of the above methods are poor in stability in the yarn forming process, and yarn breaking is likely to occur, making continuous production difficult.
Japanese Patent Laid-Open (Kokai) No. 63-211335 discloses a thick and thin yarn changing in sectional area in the axial direction by heat-treating an undrawn polyamide yarn at 110.degree. C. to 200.degree. C., to keep the crystallinity at 35% or more, and subsequently drawing at a low ratio of 1.2 to 3.0 times. However, the multifilament obtained is as long as many centimeters to several meters in the periodic length of thickness unevenness in the length direction of the multifilament, and furthermore, even though thickness unevenness can be obtained since the crystallinity of the undrawn yarn is enhanced before irregular drawing, the color shade contrast obtained when dyed is weak, and the color fastness is poor, and lowers the commercial value.